1. Field of the Invention
The present invention relates to a polymeric dispersant for hydrophobic particles including pigment particles in an aqueous system, and an ink composition comprising the polymeric dispersant. More particularly, the present invention relates to a polymeric dispersant capable of forming a stable dispersion due to its excellent dispersing properties for hydrophobic particles, and an ink composition comprising the polymeric dispersant.
2. Description of the Related Art
Hydrophobic particles tend to aggregate in an aqueous system when the hydrophobic particles are present in an aqueous medium. A dispersant is added to the aqueous system to decrease the tendency for aggregation of the hydrophobic particles.
A dispersant prevents hydrophobic particles from aggregating or precipitating, helps to break up the hydrophobic particles into small sizes during milling of the particles, and maintains the hydrophobic particles in a stable dispersed state during long-term storage of the aqueous system. A dispersant has both a hydrophobic moiety and a hydrophilic moiety. The hydrophobic moiety has an affinity for the surface of the hydrophobic particles and the hydrophilic moiety is solubilized by water thereby maintaining a stable dispersed state.
When a dispersant is added to an aqueous system, a hydrophilic moiety forms a spatial outer shell around the particles and prevents the particles from aggregating by reinforcing a negative or positive charge on its surface, while a hydrophobic moiety adsorbs a dispersant to the surface of the hydrophobic particles.
Spatial and electrostatic stabilization provided by the hydrophilic moiety occurs by powerful mutual repulsion between individual dispersant/hydrophobic particle composites. Such repulsion results from hydrophilic moieties comprising ionic functional groups. Thus, individual dispersant/hydrophobic particle composites possess effectively high charges. The ionic functional groups can be cationic or anionic, but in general should have the same type of charges. The functional groups with the same charge repel each composite thereby suppressing aggregation of the particles.
On the other hand, stable adsorption of a dispersant is needed for the long-term dispersion stability of hydrophobic particles in an aqueous system, and rapid wetting is needed for cost savings.
Over the last few years, polymeric dispersants have been proposed for use as a dispersant for stabilizing aqueous systems.
International Patent Application PCT/US1999/09145 by 3M Innovative Properties Company discloses a dispersant for dispersing hydrophobic particles in an aqueous system. The dispersant comprises a hydrophobic moiety consisting of a nonpolymerized hydrocarbon moiety linked to at least one terminal polymeric hydrophilic moiety consisting of a polymeric electrolyte. The dispersant is characterized as having a linear or branched aliphatic hydrophobic moiety adsorbed to pigment particles and a water-soluble polymer hydrophilic moiety. The dispersant having this structure does not provide stable adsorption for hydrophobic particles due to lack of hydrophobic moieties. Further, many dispersants are so selective for a pigment that specific dispersants are needed for each pigment type when used in inks.
Further, a copolymeric dispersant consisting of a hydrophobic moiety and a hydrophilic moiety is also used.
The arrangement of monomers in the copolymeric dispersant can be varied according to the preparation methods. For example, there are random copolymers in which different monomers are linked without a constant arrangement, and block copolymers in which a moiety consisting of a monomer and a moiety consisting of another monomer are linked together. The block copolymers possess properties of a single polymer consisting of each monomer in substantial portions.
The random copolymer is a copolymer in which a hydrophobic moiety and a hydrophilic moiety are irregularly incorporated. Thus, since the random copolymer has weak adsorption to the surface of hydrophobic particles, it relies primarily on an electrostatic repulsion to prevent hydrophobic particles from aggregating. Accordingly, a dispersant consisting of a random copolymer cannot ensure long-term storage stability. On the other hand, in a dispersant consisting of a block copolymer, the dispersion ability can be maximized by controlling the size of the hydrophobic moiety and the size of the hydrophilic moiety. Therefore, a dispersant consisting of a block copolymer is more effective.
On the other hand, when a block copolymer consisting of a conventional hydrophilic monomer and a hydrophobic monomer is utilized in a dispersant, the hydrophobic moiety of a block copolymer adsorbs to pigment particles. Since the moiety contacting effectively with pigment particles is actually only a part of the overall hydrophobic moieties, even in this case the desired adsorption stability cannot be obtained.
More specifically, in the dispersant consisting of a conventional block copolymer, a sufficiently long chain of the hydrophobic moiety should be ensured for stable adsorption to the hydrophobic particles. This is because it is lacking a moiety actually participating in adsorption to the surface of hydrophobic particles, among the moieties forming a hydrophobic moiety or the degree of its interaction is weak. However, when the chain is long, a viscosity of a dispersion is increased thereby decreasing various properties of materials and dispersion stability of hydrophobic particles in an aqueous system, including an ink composition.
Accordingly, a dispersant is needed that is rapidly aligned between a surface of hydrophobic particles and an aqueous medium, is not easily desorbed, and is capable of enduring drastic polarity changes.